The internal combustion engine that can execute a lean burn operation to improve a output as and the fuel efficiency together is known. A NOx occlusion reduction catalyst is widely utilized to reduce NOx emission amount of such an internal combustion engine.
Because of reinforcement of the recent emission effluent control, it is required to accurately execute a deterioration judgment of a NOx occlusion reduction catalyst. An apparatus detecting deterioration degree of a NOx occlusion reduction catalyst based on a change of the ammonia density at down stream of the NOx occlusion reduction catalyst in a rich air/fuel ratio is known (see, for example, patent document 1). According to this apparatus, the surplus reducing agent amount that was not used for NOx reduction is calculated based on a change of the ammonia density. Since this surplus reducing agent is correlated with the NOx occlusion capacity, the apparatus can detect the degree of deterioration of the NOx occlusion reduction catalyst.    Patent Document 1: Japanese patent bulletin No. 3,589,179    Patent Document 2: Japanese Patent Laid-Open No. 2002-276419    Patent Document 3: Japanese Patent Laid-Open No. 1998-68346    Patent Document 4: Japanese Patent Laid-Open No. 1996